Why don't small planes use turbine (turboprop) engines?

Question

Turboprop engines are more fuel efficient¹, lighter for the same power, mechanically simpler and consequently more reliable. They are also slightly easier to operate (no need to fiddle with mixture) and burn cheaper fuel. As a result they replaced piston engines in all transport aircraft long ago. So why didn't they find their way into small GA planes (basically everything above 8-10 seats uses turboprops, but in the 4-seat category, nothing does)?

¹_{Actually looks like I was mistaken. Reciprocating engines seem to be more efficient.}

Answer 1

First, piston engines are more efficient than turboprops, so their operational cost are lower. This also means that the system mass (engine plus fuel) for a trip is lower once you go beyond small ranges. In a helicopter, the engine mass is more important, because average flight times are much shorter, so you find many helicopters with turbo engines and only few with pistons.

Next, there is a well developed infrastructure for maintenance on those pistons, and new turboprops would need new, expensive infrastructure (trained people, tools, spare parts ...)

And then there is simply no incentive to develop a new GA engine. The expense to get it certified is too large for the rather small market. This is the same as for Diesel engines.

Answer 2

This is probably due to certification inertia, at least in some part:
A Piper Archer is certificated with a specific engine type (If you scroll down to page 21 of the TCDS you'll see it's a "Lycoming O-360-A4M"). Piper can essentially make as many Archers as they want under their production certificate as long as they conform the type certificate.

Changing the engine is a major change to the type certificate, and would require more certification test flights, paperwork, and ultimately money from the manufacturer, and sales would be uncertain (because it would be different -- A flight school flying 50 planes that are all powered by Lycoming O-360s is not going to want to take on one turbine-powered plane, it would be a maintenance headache).

It's worth noting that for some portion of the avgas-burning GA fleet efficiency gains similar to (or better than) turboprops can be achieved by converting from old-style Lycoming and Continental engines (which are effectively 1960s engine technology at best) to engines like the Rotax 912 with modern electronic ignition and engine management systems. An 80HP Rotax would pull a Piper Cub along just as well as an 80HP Continental - The barrier is again in certification (and cost to modify existing aircraft).

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On the technical end of things, a turbine engine is overkill for most small GA planes. We're talking about aircraft that usually require 100-300 horsepower (and if we're considering light GA training aircraft you're usually below 200 horsepower). The popular PT6 turboprop engine starts around 500 shaft horsepower and goes up from there.

Also while a turbine engine is a relatively simple device a turboprop gets to be pretty complex -- The turbine section is still spinning at turbine speeds, and while this isn't a problem for a jet engine on a turboprop it could easily make the propeller tips reach supersonic speeds (the "Loud and Inefficient" operating range). This necessitates a reduction gearbox to produce an appropriate shaft speed, which in turn necessitates maintenance of the gearbox.

Answer 3

First, for a given power rating, a turbine engine will be a lot lighter than a piston engine. Think about what that does to the aircraft's balance: the nose gets a LOT longer, not to accommodate the hardware put to put the weight where it needs to be. Of course, this only applies to conversions.

Second, a turbine engine will cost a lot more than a piston engine. Smaller turbines cost more per unit of power than large ones, so overall you will be paying more for the same result. The engine might use less fuel over it's lifetime but you still have to pay for it up front.

Third, while turbines in general require less maintenance, when something does have a problem the bill is not for the fainthearted. And you can cook a turbine pretty easily.

So in total it just comes down to money. If you really want one, Soloy sells a conversion kit for a Cessna 206 that will put a 450hp Rolls-Royce turbine in the nose and make you the envy of every other light aircraft owner in the region. The kit starts at $275,000. Plus engine. So you are looking at over half a million to modify an airframe that costs a bit less than that. If you have that much spare cash, pick up a matching MTT Superbike with an only slightly smaller turbine. As Jay Leno said, it's "the best ever motorcycle for shutting up the Harley guys."

Answer 4

Well, some do. Meet the Marchetti SF.260TP:

This is a very popular light single for aerobatics, which has also seen military use in CAS and observation roles. If you're a Bond fan, you'll know about this one; it appeared in Quantum of Solace. Other turboprops in the small single-engine class include:

• Piper Meridian
• One Aviation Kestrel
• Pilatus PC-9, PC-12, PC-21
• Cessna 208 Caravan/Super Cargomaster
• Epic EW1000, LT
• Socata TBM850, TBM900
• Grob G-120TP

As for why not all small singles are turboprops, the other answers have it; turboprops are less fuel efficient at the low altitudes small single typically fly at (they're great for higher altitudes) and they're a different beast to maintain. They're also more expensive; the Marchetti is about a $1.6 million dollar plane, 3 times the cost of a Piper Archer 235, though not all of that is the engine.

Answer 5

Fortunately this stereotype is changing, albeit slowly. Small GA engine development has been stuck in the back seat for generations and it's great to see a number of new start ups making the push towards change. The TP100 Turboprop by PBS is once such product. I really hope they reach their potential and bring the product to market, even if only to experimental parties. Continental is aggressively developing their new diesels and will be launching a new 230 ish hp unit in the next year. It is already in testing in China. If they get that right, it will be a Lycoming killer. They've already begun conversions with the 155 hp version in Cessnas.

One reason the lower power (150-250 hp) turbines have not been developed more rapidly is that GA pilots have not pushed for them. They know they take more fuel, even if the power/weight ratio is far superior. The operating costs make them hide in the corner.

RR/Allison made an effort to refresh the aging 250 with a modern turbine that would appeal to the Cirrus crowd but bowed out when the 2008 financial crisis hit. The problem with companies like Allison is their development costs are through the roof.

Change in this space is going to require guts from the little ducks of the world. Many of which are now starting to appear on the scene. There is a well know shortage of 100LL around the world and soon here in the US. That and the high price of carting this boutique (not shipped in pipelines) fuel will fuel the market for these new products.

There is no rocket science at work here. It's old technology (albeit with a bit of electronic intervention) that just needs a market to grow in. Today we are moving towards that market. Get used to it and consider tossing that ole' Lyc.

Answer 6

Another thing to remember about the specific fuel consumption of a turbine engine is that it is HIGHLY dependent on the size of the engine. Small gas turbines (of the sort that may be used in small private aircraft) are inherently inefficient because they cannot run the kinds of pressure ratios required for high efficiency. For instance, a 2180HP PW119B has a pressure ratio of 13.2:1 and a SFC of .49. Step that up to an 11000HP Europrop TP400-D6, and pressure ratio climbs to 25:1. SFC drops to .39. And the 56300HP GE LM6000 (a marine and power generation turboshaft based on the CF6 turbofan) has a pressure ratio of around 32:1 and a SFC of .32. Because of this, piston engines have a fuel efficiency advantage at low power outputs, but a fuel efficiency DISadvantage at high power outputs. Piston engines therefore are favored in small aircraft for this reason and initial cost. But turbines make MUCH more sense in larger aircraft due to both their higher efficiency than large piston engines and reduced maintenance costs.

Someone mentioned helicopters here. And helicopters are a different animal altogether. A basic characteristic of helicopters is the fact that they require LOTS of power on a continuous basis. This would take a HUGE toll on piston engines in terms of reliability (think of how long a car engine would last if you constantly drove it at full throttle), but is not an issue for a turbine. Turbines are also smaller and lighter (ever seen a Sikorsky Mojave with its ridiculously huge piston engines)? All of these factors make turbines MUCH more practical than piston engines in helicopters.

Answer 7

I'd say choice between a turboprop or a reciprocating engine may depend on your power needs, and the type of 'mission' the engine/aircraft will endure.

Turbines and Turboprops, even with its advantage of weight to power ratio, are not that good regarding SFC, if you check this in www-jet-engine.net civil turboshaft-turboprop specifications, most of it have an SFC in the range of 600 lb/shp/hr, or more.

The Harry Ricardo and Roy Fedden Single Sleeve-Valve distribution Aircraft Engines provided the best ever SFC in a gasoline aircraft engine: 0.42 lb/BHP/hr in an Hercules Centaurus, 3272 cu. in., with a weight of 2695 lb, for a power of 3150 HP. Even more impressive results were obtained with an Open-Sleeve, acting as an annular piston with 10% of piston area, transmitting 3% of power, in an experimental 2-Stroke Compression Ignition Engine by Harry Ricardo. No Sleeve-Valve engine is produced today, but new replacement sleeves for old engines are still manufactured, they said this in AEHS, but I failed in locating the seller of Sleeves for old SSV engines.

Best SFC ever seems being obtained in an open-sleeve valve, two-stroke, compression ignition engine, by Harry Ricardo, at 0.34 lb/HP/hr ('The high speed internal combustion engine', 1968 ed)

A poppet valve engine of the times of Bristol Hercules, the Wright R-3350, 2907 lb weight, also air-cooled, with same number of 18 cylinders; 3348 cu. in.; 2800 HP, used 0.72 lb/HP/hr, but this was greatly improved in the Turbo-Compound units, that added 500 lb weight more.

The Sleeve-Valve engines were known for its reliability and low wear, the no-lubrication zones of TDC and BDC are wiped out by the continuous sleeve movement, the Bristol Hercules had a TBO of 3000 hr or more, for 2000 hr TBO in the Wright cited (data in AEHS and other sources). Sleeve-valve engines, having no hot spots in combustion chamber, can work on low octane fuel at higher Compression Ratios than poppet valve engines.

The experimental Single-Cylinder, 500 cc, Sleeve-Valve gasoline unit built by Mike Hewland for Automobile use was reported having an SFC of 0.45 lb/HP/hr in the racing version, and 0.39 lb/HP/hr in the economical version (Car&Driver, July 1974), working even with creosote.

A Rolls-Royce Turboprop, the Dart RDa 10.1 series, with 2915 HP and a weight of 1207 lb, consumed 0.550 lb/HP/hr.

You can't find today reciprocating engines in power ranges above a certain limit.

The Wankel Rotary Combustion Engine, that had extensive development for aviation and other uses by Curtiss-Wright, John Deere, NASA, NSU-Citroën, Sachs, Aixro, Rolls-Royce, Mazda, had an SFC of 0.46 lb/HP/hr with a weight of around 210 lb for a power output of some 120 HP, street car version, as reported in an early Mazda NA Wankel Engine, 2 Rotor, liquid cooled, a Wankel will work with no toil with unleaded gasolines around 80-90 ON, also with 10% Gasohol, that provides lower SFC and reduced operation temperatures and thermal load, some added lubricating oil to fuel and working chambers is always used in Wankels, no need of changing lubricating oil.

A major breakthrough was achieved recently in the Florida University, where they proved that adding: 'Heat Pipes', for cooling the housing and side plates of a formerly air-cooled housing charge cooled rotor UAV UEL engine reduced maximum temperature to a mere 129º C, and maximum temperature difference between engine parts to 18º C, the use of: 'Heat Pipes' for cooling Wankel RCEs, as it were used in satellites, may be an enormous step forward in Rotary Combustion Engines, as it eliminates most if not all thermal dilatation differences between parts of engine, making design and construction much easier, also improving reliability, from a reduced wear, enhancing power, reducing emissions and boosting fuel economy. Who could ask for anything more?

Standards for reciprocating aviation engines were around 1 kg/HP weight to power ratio, and 250 gr/HP/hr SFC.

No Wankel Rotary Combustion Engine seems having obtained yet an FAA certification for regular general aviation use, even when it are considered safer than reciprocating engines, Wankel RCEs failures tend not being total and instant as in reciprocating engines, a residual temporary lower power working would allow for more safe landing opportunities, this safety concern was approached by something close to the basic concepts in an hybrid car by Axteraerospace.com

This one is an example of Turbine for General Aviation build in Argentina, basic arrangement reminds the first Turbine by Hans Joachim Pabst von Ohain

Answer 8

The cost impact of a turboprop has been underestimated in many of the earlier answers.

Turbine rotors are subject to very high temperatures while turning at very high rpm; the G-force on a turbine blade is enough to make your eyes water. Due to a minute manufacturing fault or operational error or metal fatigue, one blade lets go; then it will tear through the steel engine case, through any unlucky oil or fuel plumbing, then through the aluminum cowl. The manufacturer will spend hundreds of thousands of dollars to prevent this from happening, and must recover that cost from engine sales.

And that's just from one broken blade, weighing maybe a few ounces.

If a much-heavier rotor disc breaks from fatigue, it could saw the airplane in two.

So yes, a turbine burns cheaper fuel, but more gallons than a recip. You can see there are pros and cons!

Answer 9

Actually they do have a number of smaller turbine powered aircraft these days. And turbine STC conversion kits for existing GA aircraft are popular - the P210 Riley Rocket quickly comes to mind as does the PA-46 JetProp plus high end experimental aircraft such as the Lancair Evolution and IV-P Turbine. These turbine conversions give all the advantages listed above, but at a higher fuel burn.

The primary deterrence against an entire turbine GA market are the higher operating costs and excessive performance for a neophyte or casual flyer.

Answer 10

Matt got a lot of it right.

Dynamic compressors and turbines tend to get less efficient as their sizes decrease, because of Reynolds' number effects and because of limits on materials and manufacturing. To get a good stage pressure ratio in a compressor, a tip speed of at least 300 m/s is required; exceeding 500 m/s is not unheard of. Since the centripetal acceleration is inversely proportional to radius at constant tip speed, centripetal loads can be too high for most materials. Tip clearances have to be small -- about 1% of tip chord -- for best efficiency, high temperatures demand cooled blades and vanes in the hot end, and that means that blades and vanes have to have cooling passages inside the blade. This is easier in a PW4084 than an Allison 250, as the PW's blades are a lot bigger.

Answer 11

It is much more expensive to build a turbine engine than a piston engine. Turbine engines can be built that have a longer TBO than a piston engine, partially offsetting that cost, but that matters more to users who have high aircraft utilization per year. One tends to see turbine engines used in commercial operations, 1000s of hours per year, and pistons in private operation 100s of hours per year. As engines get larger and more powerful, piston engines get more expensive and complex because they are limited by engine knock. So turbines dominate at larger sizes for all users. It is all driven by dollars. No bucks, no Buck Rodgers.

Answer 12

About the questions of knock limiting the size of piston engines and 107000HP marine diesels, knock is not an issue. Rather, the limitation on the size of piston engines is PHYSICAL SIZE. Reliability is also an issue as the number of reciprocating moving parts skyrockets. But size is what REALLY kills the deal when it comes to piston engines. ln my other post, I mentioned the Sikorsky Mojave. That helicopter looks downright bizarre because of the large pods protruding from each side of the fuselage. Those pods each contain a 2100HP Pratt and Whitney R-2800. By comparison, the 5000HP Honeywell T-55s on a Chinook look almost like toys. If you wanted to power a Boeing 747 with piston engines, each engine would probably have to be the size of a decent sized plane in order to produce the ~55000-60000HP that each CF-6 produces (consider how big the R-4360, the largest production aircraft piston engine was, and it topped out at around 4000HP). The 107000HP marine engine is an extreme example of this. These engines are literally called 'cathedral engines', and live up to that name with 38 inch bores. Yes, it may produce about as much power as two CF-6s. But these engines are as large as they sound, and not exactly aircraft material. On the other hand, size is not so much an issue in an ocean going ship. And reliability is not too bad for these engines since the large size means that a smaller number of super large components can be used.

Answer 13

Small aircraft dont need the only advantage turboprop allows: more power to weight ratio. A turboprop is more combersome than a piston engine for lower power to weight ratios It will not save so much weight comparing to a small displacement piston engine since a lot of the weight comprises other parts of the propulsion system beside the engine ( gears, fuel systems, cooling, charging system, propeler, etc), and a turbine allways consumes more fuel.

Answer 14

For me, the answer is money. A P&W PT-6 turbine engine is going to run you between \$500,000 USD and \$750,000 USD to be overhauled. A turbocharged piston engine, even a very expensive one, isn't going to cost more than about \$50,000 USD to overhaul. Other than that, for certain missions such as over-water/mountain operations I'd take the fuel burn for reliability and weight savings trade-off in a small plane, though you can expect to burn 2 to 3 times more fuel in a small turbine vs a large piston. Note that there are many piston types that have STCs available to replace the standard piston engine with an aftermarket piston, but as I mentioned the costs are relatively astronomical to do so. For example, the Silver Eagle Cessna 210. http://www.onaircraft.com/the-planes/the-silver-eagle-ii/